texts:
In the New Scientist article Is this ET? Mystery of strange radio bursts from space, it is reported that several times since 2001, astronomers have detected fast radio bursts that seem to have a frequency of multiples of 187.5.  In the article, the theory that it could be a pulsar is discounted.

Are there any astronomical phenomena that could emit strong radio waves with multiples of a discrete frequency?


texts: How long would an occultation by a TNO last?

texts:
A trans-Neptunian object, 1 million km from the observer, with an angular diameter of 0.126° occults the Sun (angular diameter of 0.004°) and the TNO and the observer are moving in the same direction, in the same plane.  The TNO is moving 8m/sec with respect to the observer, how long would the occultation last?

Assume the observer is 18 billion km from the Sun.


texts:
Two white dwarfs with a total mass $>1.4\:M_{\odot}$ merge. It may trigger a type Ia SN explosion.

How about the three cases below?

Two low mass WDs merge, total mass $<1.4\:M_{\odot}$.
A low mass WD + a low mass neutron star, total mass $<2\:M_{\odot}$ but $>1.4\:M_{\odot}$.
A low mass WD + a neutron star, total mass $>2\:M_{\odot}$.

If any of the above can result in a SN, have we observed this kind of SN?


texts: Does the Perseid peak cause enough light pollution to be a problem?

texts:
The annual Perseid peak occurs between August 9-14. During this time, about one meteor burns up in the atmosphere per minute.

When these meteorites burn up, they generate light. When you have one of these per minute, does it produce enough light pollution to be a problem for those making astronomical observations?


texts: A quick (elementary) check on arsecond conversion to sky distance in parsecs

texts: I just wanted to double check I had something correct. If an object, say a nebula, is measured to roughly have a diameter or extent of, say, 250 arcseconds, would its diameter in parsec simply therefore be 250 parsecs?


texts: By putting a mirror in space, would we be able to see into the past?

texts: I only ask this because of how fast light travels. The question remains in the title. Why, or why not, would this work?


texts: How is it known that Callisto has no core?

texts:
My Astronomy book claims that scientists have discovered that Callisto, a moon of Jupiter, has no hot inner core. In fact, it says, Callisto has a core much like the nucleus of a comet.

Is this still accepted theory, and how was this information discovered?


texts: It came to my mind one day that why does this universe even exist and why do we even exist and like we are enclosed in the so called atmosphere and a floating rock in space called the Earth, so is there any possibility that the universe is also enclosed in something and if it is, then what is outside that enclosure?


texts: Is it possible for a planet's axis to precess or wobble in such a way that its season cycle is of lesser duration than its year? The gyroscopic precession will thus give seasons to one or the other hemisphere during a possibly hundreds-year-long revolution around its star. Earth's precession is very much longer than it's revolution period, but is it possible that these could be reversed and a planet still be habitable?


texts: galaxies in a specific radius in different shapes of universe

texts: Let the distribution of galaxies be a number like $n$. we want to find the total number of galaxies within the radius $r$ from a specified point. in which shape of the universe (flat, open, closed) the number we get is higher? ($n,r$ are the same in this universes and only the area (volume) in each universe is different).


texts: Where can I find a catalog for stars within 450 light-years of Earth? I'd prefer one that has approximated masses as well.


texts:
Researchers saw repeated outbursts — every 114 days, on average — in a galaxy some 570 million
light-years away from Earth. Here outburst because of black hole draws mass off from the galaxy?
Location: Galaxy ESO 253-3.
Why does black hole draw off some mass from orbiting star instead of sucking it? As we all know black hole sucks things instead of weird behavior like this.


texts:
I'm very new to astronomy and wondering what I can see with these specs, I honestly have no clue what a lot of these specs mean, and need help thanks.

Features:

Maximum magnification: 675x

Large aperture offers clearly illuminated views

Stable construction provides solid support and control

Metal tripod with slow motion control rod

Strong performance in both high and low magnification

Easy sky focusing with finderscope

Specification:

Aperture: 60mm (2.36inch)

Focal Length: 900mm.f/15

Finderscope:  5x24

Diagonal Prisms: 90degree

Slow Motion Control Rod For Easy Vertial Micro Adjustment

Height: 128cm

Standar: 0.965” Acceddoness Include

Eyepiece: SR4mm H12.5mm H20mm

3X Barlow Lens, 1.5X Erector

Telescopic Aluminum Tripod

it also cost $99, and also wondering if it's worth for the price
here's a link to the site

http://www.dshop.com.au/buy/astronomical-telescope-675x-magnification/F90060M?gclid=CjwKCAjw-qbLBRB7EiwAftBCI7h_7lH3E9goJG4Top0N-OHaraxB0cM8IiZgfygRkHD8DZ1ThARCdRoCzA8QAvD_BwE


texts:
What exactly is meant by spurious periods of stars? E. Tanner writes in Spurious Periods in Spectroscopic Binaries in 1948

Furthermore: Can spurious periods be produced by aliasing, and how can they be handled? I read about this that these periods can be detected while using Robust variogram estimator in this paper.


texts: What is the Inner Horizon of a rotating black hole?

texts:
My understanding is that any object with non-zero angular momentum which undergoes gravitational collapse to a black hole will become a rotating black hole. In reality all black holes will be rotating black holes because all objects undergoing gravitational collapse will have some angular momentum (even if it is relatively small).
I understand that the inner structure of a rotating back hole contains:its boundary i.e the event horizon, the “inner horizon” and the singularity which is ring-shaped. Once an object crosses the event horizon it has passed the point of no return and the singularity is a one-dimensional ring of infinite density. However,  I’ve been unable to locate  an article explaining exactly what the inner horizon is which is understandable to someone who has some mathematical background, but without a degree-level knowledge. For example, entering “Inner Horizon” into Wikipedia redirects to the following:
https://en.wikipedia.org/wiki/Cauchy_surface#Cauchy_horizon
Which with my lack of detailed mathematical background is incomprehensible!
I have found that many of the popular science articles about black holes make statements which are unsatisfactory and  and raise a lot of other questions .
E.g. https://www.discovermagazine.com/the-sciences/peering-inside-realistic-black-holes states
“To visualize the inner horizon, consider this thought experiment: Imagine you could enter a spinning black hole and cross the event horizon while holding a flashlight that’s pointed outward. That light would eventually freeze along, or within, a spherical surface. That’s the inner horizon, ….. the point where the black hole’s gravity is strong enough to keep light from leaving, but not strong enough to drag it all the way into the center. The trapped light becomes so energized that it turns into a firewall of radiation, incinerating anything that passes through it.”
Does anyone know of an article explaining exactly what is the inner horizon is to the educated layman? Or would be able to explain themselves  by answering this question? I would like to know what happens when an object crosses the inner horizon and WHY?


texts:
I notice that Wikipedia give two contradictory terms for the time period in which a planet rotates in relation to an infinitely-distant object. In one place this is called a "sidereal day" and in another "stellar day". Which of these are correct? For that point, is this an IAU definition? I cannot find the definitions on the IAU website.

From the Sidereal Day page:

  A sidereal day is the length of time it takes a planet to rotate from the perspective of a distant star.

From the Day page:

  stellar day - an entire rotation of a planet with respect to the distant stars
  sidereal day - a single rotation of a planet with respect to the vernal equinox


texts: When are the planets occulted by the sun?

texts:
There are numerous lists available of past and future planetary transits and occultations, but I can't find any that list transits behind the sun.

Do the planets in our solar system ever pass directly behind the sun, from Earth's perspective? Is there a special name for this kind of occultation, and is there a list of these events?


texts: How could lithium burning take place in a quasi-star?

texts:
According to Begelman et al. (2008), one of the most distinguishing features of the hypothetical quasi-star is that it's supported by radiation pressure from the accretion disk of the black hole in its core. The paper suggests that these stars form from Population III stars.

What's more, while almost all of the star's luminosity is from the accretion disk (which is far more luminous than thermonuclear reactions), temperatures around the central black hole are hot enough for fusion:

  Although we do not model the non-hydrostatic region of quasi-stars in any detail, in our case high temperatures are attained in the immediate vicinity of the black hole. However, even in this region the neglect of nuclear reactions is justified, first because black hole accretion is energetically much more efficient than fusion, and secondly because on-going accretion limits the time-scale over which inflowing gas is exposed to high $T$.

That's all fine, but what surprised me is the footnote on the third page:

  In the most massive quasi-stars, the central temperature may be high enough (a few million K) to initiate lithium burning. This is energetically negligible, and although the presence or absence of lithium does affect the opacity, the effect is small for the photospheric temperatures and densities of interest here.

Even though this part dismisses lithium burning as an important factor, since it's energy output doesn't compare to that of the accretion disk's, it still mentions that lithium burning can take place. Considering that these form from Population III stars, how is it that much lithium burning can happen in the first place?

Although lithium is a relatively light atom, it is considered a metal and lithium burning depletes the amount of lithium in a star. Since Population III stars are supposed to be nearly metal-free, shouldn't only hydrogen or helium fusion be a factor here, even if they only have a small effect?


texts:
We have apparent magnitude of stars, planets, DSOs. What about the apparent magnitude of Milky Way?
Can it be estimated?


texts: What does the surface of Mercury look like?  What color is it (I have seen some images portraying it grey and some a light brownish color), and would the surface be dusty like the Moon, smooth, or even cracked (e.g. see image below) due to thermal expansion and contraction?


texts: What's the rationale behind the false colours in solar observation photographs?

texts:
Images from SOHO, SDO and other sun observatories are often coloured differently for different wavelengths or temperatures:

http://www.nasa.gov/mission_pages/sunearth/science/Sun-Wavelength-Chart.html has statements like "This light is emitted from the upper transition region and the chromosphere and are typically colorized in red."

How is the convention of which temperature or wavelength gets which colour established?

Is it simply so that scientists can immediately tell what temperature or wavelength a picture shows, or are the colours optimized to show particular features?

And why is there a white ring or circle in the sunshade in some SOHO imagery, and why different sizes compared to the disc in the two photos above?


texts: Does any iron fuse in stars before they go supernova?

texts: I understand that iron and all heavier elements consume more energy to produce than they make, and that is what eventually leads to a supernova. I also understand that a lot of the heavier elements are produced during that supernova. However, what I'm wondering is, before the star goes supernova, does any of the iron fuse with other elements? Yes, there would be a net energy loss, but if there is only small amount of iron in the star, it would probably be able to handle that.


texts:
I was messing around with 4-body problems, and discovered a system where a close binary and a regular binary pair orbit each other. The regular binary gets disrupted every few orbits and one star orbits the close binary, while the other orbits farther out. Then, as the outer star reaches periastron, the inner star gets captured by the outer star and the cycle begins again, shown in this video (the stars here are color-coded).
Is this possible and is stable in the long-term (without any external perturbations), and have we discovered anything similar to this?


texts: How long will it take to reach the closest earth like planet

texts: Using the engines today how long will take humanity to reach earth like planets?


texts: What does it mean to say 'space exists?'

texts: From another question I posted, I learned that not only was matter created from the Big Bang, but more importantly, space itself.  What does it mean for space to be created?  Does it exist in the same way substances exist?  In other words, what do we mean by 'space exists?'


texts: It's 5.50 am now in Dhaka, Bangladesh.  I just saw a moving star around 5 am. It was a starry sky and that star was moving in a straight line. It was very fast, faster than regular planes (though I didn't hear any sound) and disappeared in the Eastern sky.  It was a little brighter and a little closer than the other stars. Can anyone tell, what was it?


texts: How would one estimate the rotation period of a star from its spectrum?

texts:
The figure is shown; the measurements were taken on two consecutive observing nights. The Ordinate is the flux normalized to continuum and the abscissa is the wavelength scale. You can see the "bumps" indicated by the arrows referring to some Starspot as the spot moves on the profile; assuming a single time-stable position-stable spot.
The "bumps" slightly shifts, as indicated by the arrows in the top line profile compared to the bottom profile, as the spot "moves across" the surface as the star as it rotates; i'm just not sure how to get an estimate of the rotational period from this? Given just the wavelength for the abscissa.
Figure reference : Gray, D. (2005). The Observation and Analysis of Stellar Photospheres (3rd ed.), page 498. Cambridge: Cambridge University Press. doi:10.1017/CBO9781316036570


texts: What is the average Ni-Fe crystal length in iron meteorites?

texts:
Iron-core meteorites display an awesome criss-cross pattern known as the Widmanstätten pattern, shown below:

(photo by Dr. Patrick M. Len, taken from Wikipedia)

I TA for an intro-level astronomy/planetary science class, and today we discussed meteorites. A student asked me what the average length of these crystals is for iron meteorites, but I hesitated. The samples we had available (and the image above) made it look like the average length was on the order of a centimeter, but with great variation. I'm curious as to whether there is a more quantitative answer.

My question:

How long is the average length of a given linear crystal in a typical iron meteorite? How much variation is there from sample to sample?


texts: What would be the difference in distance between earth and moon on supermoon night (14 November 2016)

texts:
As we all know the usual distance between earth and moon is 384,400 km.
But i was thinking how much closer it will be on 14 November 2016, as it will be a supermoon, the brightest and biggest moon in 60 years.


texts: What's the estimated remaining life of Halley's Comet?

texts:
I know that Halley's comet is seen from Earth every 76 years or so since a couple centuries. As it travels, and particularly when it comes near the sun, it loses some of its material (ice). Someday, all of this material will be exhausted and the comet will disappear (or explode I think).

Is there an estimation of how long the comet still have to live ?


texts: I have heard about space-time fabric but I can't understand its definition. Is space-time fabric a physical thing or did Einstein imagine it to assist him with general relativity?


texts:
I am looking for a raw radio-astronomic antenna array dataset of relatively modest size (on the order a few MB, or even less), to evaluate a data processing algorithm. The dataset ideally should be simultaneously sampled received signals from several antennae with known positions, containing a number of sources differing in direction of arrival and/or frequency.

Perhaps, some python library with standard algorithms for building a 2D "synthetic aperture image" of radio sources might have bundled example data, which would satisfy me. I have come across the LOFAR database, which seems huge and requires complicated processing pipeline, so my question may be along the lines of "how to get a small drink from a firehose".


texts: Are lunar occultations visible to the naked eye?

texts:
The Moon occults a lot of stars. But are these events visible to the naked eye? Won't it be blinded out by the crescent even at its thinnest just before sunrise? Did ancient astronomers actually observe them, or did they deduce them theoretically according to orbital calculations? (I suppose occultation during an eclipse is both invisible and unlikely, right?)

Venus, the brightest planet, like the new Moon always stays near the horizon. But could one actually see even Venus disappear behind the lunar anti-crust? A popular medieval islamic symbol suggests to me that they were well aware of this phenomena. Both the Moon and Venus are still symbolically important in their culture. But was occultation directly visible or inferred?


texts:
In our Solar system, the density of major planets varies from 0.7 g/cm3 (Saturn) to 5.5 g/cm3 (Earth).

For exoplanets, the density varies from very low (0.03 g/cm3 for Kepler-51x) to very high (77.7 g/cm3 for Kepler-131c, data according to NASA Exoplanet Archive).

We know that bodies with low density are composed of gas and others are rocky planets. Wikipedia provides some approximations about it.

Is there any detailed classification of planetary bodies by density? A classification that would say, for example, that the density below 1.5 g/cm3 corresponds to a gas planet; between 1.5 and 3 are icy; between 3 and 8 are rocky; above 8 g/cm3 are even more rocky etc...

Would such a classification make sense?

If it exists, which are the numeric bounds for the different types of bodies?


texts: What causes random irregularities in the earth's rotation accounted for by dynamical time

texts: I read in a book that the earth's rotation sometimes shows unpredictable irregularities(not caused by precession, nutation, tidal forces) which are accounted for using dynamical time. What would be the cause of these?


texts: What will be the temperature on Earth when Sun finishes its main sequence?

texts:
We know that presently Sun is 4.5 billion years into its main sequence. It has another 5 billion years before it enters the Red Giant phase. We also know that Sun's luminosity increases by 10% every billion years during the main sequence. I am interested in finding the temperature rise as we approach the end of main sequence. I got two different values for temperature on Earth.

Wikipedia entry says that temperature on Earth would be 422 k in 2.8 billion years. However, if we use the formula for effective temperature as discussed in this answer
https://earthscience.stackexchange.com/a/4274/15299 and L = 1.8 then, the temperature on Earth would be 330K. Also in this book, the author does the same calculations on page 255.


texts: Why is the equinoxes only 2 days and not more?

texts:
The earth is tilted at 23.5 degrees which means that in the solstices the north/south hemisphere is more radiated by the sun and the tilt to the sun is maximum.

In the equinoxes, the tilt is not towards the sun but to the "side" so the equator has direct exposure to the sun. As a result, we have 12 hours on day and night on the equinoxes.

What I don't understand is why is it only for these 2 days (equinoxes) that this happens. In the next day(s) I think the equator should have the same exposure to the sun as the equinoxes. Has the tilt changed so much?
I couldn't find any animation that shows the exact path of the tilt throughout the year.
Can someone please help me understand this?


texts:
I usually think of SOFIA, the Stratospheric Observatory for Infrared Astronomy as an infrared optical telescope:

SOFIA uses a 2.5 m (8.2 ft) reflector telescope, which has an oversized, 2.7 m (8.9 ft) diameter primary mirror, as is common with most large infrared telescopes. The optical system uses a Cassegrain reflector design with a parabolic primary mirror and a remotely configurable hyperbolic secondary. In order to fit the telescope into the fuselage, the primary is shaped to an f-number as low as 1.3, while the resulting optical layout has an f-number of 19.7. A flat, tertiary, dichroic mirror is used to deflect the infrared part of the beam to the Nasmyth focus where it can be analyzed. An optical mirror located behind the tertiary mirror is used for a camera guidance system.

and one thing that makes that happen is that it looks just like a "normal" telescope with shiny polished and metal coated mirrors.
I've just seen Phys.org's Stellar feedback and an airborne observatory: Team determines a nebula to be much younger than previously believed (which links to Stellar feedback and triggered star formation in the prototypical bubble RCW 120) which talks about a "1.9-THz fine-structure line".
As far as I can tell this is done with little antennas as pixels, a heterodyne down-converter and radio receivers just like the Atacama Pathfinder Experiment data at 345-GHz it compares to.
In other words, it sounds just like a low granularity focal-plane array on a dish radio telescope.
I am not asking about the difference between the overlapping labels of radio and infrared as they apply to the electromagnetic spectrum, though that could well be part of an answer. These days radio astronomers call what they receive "light" as often as optical astronomers do. However the equipment used is different. Radio offers the possibility of recording phase for off-line interferometry, whereas optical detector arrays are generally intensity based only. So instead, am simply asking:
Question: Is Sofia a radio telescope proper?

Source

A) SOFIA [CII] integrated intensity, scaled from 0 to 260 K km/s. The red circles indicate the approximate inner and outer PDR boundaries defined from Spitzer GLIMPSE 8-μm emission (55), and the red star shows the location of the ionizing source, CD −38°11636. The yellow “+” indicates “Position 1” (see the Supplementary Materials). (B) Spitzer GLIMPSE 8-μm emission. The contours are of [CII] integrated intensity, scaled from 40 to 160 K km/s in 40 K km/s increments. (C and D) APEX 12CO(3-2) and 13CO(3-2) integrated intensity, scaled from 0 to 260 K and 0 to 90 K km/s, respectively. The contours are the same as in (B). The areas enclosed by the dashed yellow lines in (A), (C), and (D) were used to extract the position-velocity diagrams shown in Fig. 2.


texts:
I've got a file which contains data about photons coming from some pulsar. For each photon I know: a) time, when it was registered, b) a probability that this photon came from pulsar but not from galactic background, c) energy of a photon. My task is to find a period of pulsar.
I suppose I have to use FFT to find a period but there are two issues. First of all, there are lots of photons with low probability (background photons), so I have to somehow select relevant photons. Second, I've got an unequally spaced data, so there is a problem with using FFT.
I am a novice in data processing. Could you please give me links to some lectures or books where this topic is discussed clearly?


texts:
Since I learned about Newton's gravity law, I wonder if I can measure the moon's gravity using (almost) no special equipment. The best I could come up with is a pendulum, which is in a resting position. The moon (at the horizon) will pull on the weight at the bottom of the pendulum and cause it to shift. 6 hours later, with the moon above or below the pendulum, the weight should be back. Maybe we could detect the shift, just like we see the tides.

Running the math for a 5 meter long pendulum and 1 kg mass, I get an amplitude of 0,017 mm. I fear that external forces (wind, unstable surface, ...) will be bigger than this effect.

So my question is: is it possible (again, without specific equipment) to measure or detect the moon's gravitational force? What would be the best way to measure/detect it?


texts: Why does Nancy Grace Roman = 100 × Hubble? Why is the new space telescopes wide field camera so much wider than the old one's?

texts:
The title of the WFIRST project description (before it was named the Nancy Grace Roman Space Telescope) is The Wide Field Infrared Survey Telescope: 100 Hubbles for the 2020s.
Question: Why does Nancy Grace Roman = 100 × Hubble? Why does the new space telescopes wide field camera so much wider than the old one's?
The telescopes have very roughly the same wavelengths and apertures, so does the new one have one hundred times more pixels at the same resolution, or 100 times larger pixels? Is there better correction optics that allows for a flatter field for a flat array, or a curved focal plane array, or other optical correction?


texts: What alternative facilities would be available in the event of JWST being destroyed?

texts:
Unfortunately, space launches can and do go wrong. Suppose that after all the delays and budget overruns, the launch of the James Webb Space Telescope fails and the telescope becomes a cloud of very expensive shrapnel raining into the sea.

Are there any upcoming/planned facilities that would be able to do similar science to what JWST would be capable of within the next decade, or are all our eggs in one basket on this one?


texts: Apsidal Precession: What Am I Getting Wrong?

texts:
In Precession of Mercury’s Perihelion from Ranging to the MESSENGER Spacecraft (https://ui.adsabs.harvard.edu/abs/2017AJ....153..121P/abstract), one finds the precession of Mercury’s perihelion due to the oblateness (quadrupole) of the Sun to be:
$ \displaystyle \dot \varpi_{J_2} = \frac {3}{2} \frac {nJ_2}{(1-e^2)^2} \left( \frac {R_\odot}{a} \right)^2 \left(1 - \frac {3}{2} sin^2 i \right) $
(equation 3, p. 2). A few lines down, we read that it amounts to about 0.03″ per century—later refined to 0.0286″ per century.
However, when I do the calculation, with $ a $ = 57.90905 Gm, $ e $ = 0.20563, $ i $ = 3.38°, $ R_\odot $ = 696,342 km (all four from Wikipedia), $ n $ = 4.09°/d (calculated from formula 33.6 in Meeus 1998), and $ J_2 $ = 2.25 × 10⁻⁷ (paper’s abstract), I get 0.01855″ per century.
What am I doing wrong?


texts:
Let's assume that there is a perfectly spherical planet and there is a moon which is also perfectly spherical. Let's assume that there is no atmospheric drag and no other gravitational pull. If the moon is put in a perfectly circular orbit around the planet somehow, would the moon eventually "fall" towards the planet and form an elliptical orbit or would it continue to follow the perfectly circular orbit?

Edit: What i actually wanted to ask is that would the gravity of the planet cause the moon to "fall" towards the planet or would the gravity allow the moon to continue its orbit without bending its path any further towards the planet. I know that no planet can be a true sphere or a cube due to the shapes of the particles.


texts: Did Tycho Brahe improve measurements twentyfold?

texts:
I am reading a book on the Coopernican Revolution where the author states that Tycho Brahe was able to make twenty times more accurate measurements than any of his predecessors.

On the other hand, I remember having heard a lecture where the speaker said that Tycho did not significantly improve measurements since Ptolemy because no new equipment (telescope) had been invented.

So, which statement is correct?


texts: Why does the eclipse in this video look annular?

texts:
In this article about the AS870 flight that flew through the umbra of the March 8-9 2016 solar eclipse, there is a link to a YouTube video showing the eclipse recored by a handheld camera in the cabin.

Of course this is personal video and not recorded for analysis, but the images - especially during the zoomed part - remind me of an annular eclipse, rather than a total eclipse. Below are some screenshots - the weak Fresnel reflections in the cabin window act a bit like attenuators (ND filters) and give less saturated, though distorted views. Basically they all look like donuts.

My first thinking was that if it was a total eclipse on the ground, it couldn't be annular at 35,000 feet closer to the moon. But thinking and looking again, the sun is near the horizon, and the plane at about 150 West longitude is not necessarily closer to the sun than observers on the ground were in Indonesia circa 120 East.

My question is - is that just corona, or is the plane farther away enough from the moon at this time and location to make the eclipse annular?

It may be helpful to read about hybrid eclipses.

note: I'm looking for a quantitative answer, not an opinion.

There is another version of the video on YouTube with more views, but the narrative doesn't hold a candle to double rainbow guy original and musical version.

Here is a screenshot of a typical AS870 flight from flightradar24.com, pretty much between 152 and 158 degrees west.

This is from the second link in the top line - the Alaska Airlines blog entry for the flight.

How an eclipse could possibly appear annular from a plane over one place on the planet while appearing total on the ground at another place on the planet 5,000 kilometers away (note, these occur at different times - the shadow is moving relative to earth and the earth is rotating):


texts: What have we learned from the observation of most distant galaxies

texts: We have seen galaxies around 13 billion years old. Since these galaxies formed so early in the history of the known universe, are there observables about these galaxies that are apparently different from much younger galaxies. I suppose all current physics laws had condensed out by then but was just curious about the formation and 'aging' of such old galaxies.


texts: How did the amount of nuclide produced by s- and r-process changed over time?

texts: For a exercise I need to find out how the amount of nuclide produced by s- and r-process changed over time and I also need to draw a schematic diagram of the s-/r-ratio vs. time from 13.6 Ga till today and explain my reasoning. I searched for an answer in my lecture notes and also the lecture but I can't find a solution for this problem. Can somebody help me out where I could find an answer for this?


texts: Is there any possible connection between Gamma Ray Burst and Fast Radio Bursts?

texts:
The physical nature of Fast Radio Bursts (FRBs), a new type of cosmological transients discovered recently, is not known. It has been suggested that FRBs can be produced when a spinning supra-massive neutron star loses centrifugal support and collapses to a black hole.

Here we suggest that such implosions can happen in supra-massive neutron stars shortly (hundreds to thousands of seconds) after their births, and an observational signature of such implosions may have been observed in the X-ray afterglows of some long and short Gamma-Ray Bursts (GRBs).

Under this picture, a small fraction of FRBs would be physically connected to GRBs. Is there any possible connection between Gamma Ray Bursts and Fast Radio Bursts ??


texts: Does nucleosynthesis happen near a black hole or during its formation?

texts:
During its lifetime, a star can synthesize elements to iron because of the pressure and temperature. A neutron star is composed of iron nuclei 56 and up to krypton 118.

According to that, a black hole (which concentrates matter to an increasingly tiny space during infall) must also have an unimaginable pressure/temperature during its formation and as additional matter falls into it.

Can it therefore synthesize new elements?


texts: In which countries are there organised Messier Marathons?

texts:
Just out of curiosity...

In which countries have there been or are current, organised Messier Marathons?

Thank you for all answers and effort for this!


texts: Could dark matter be considered a medium?

texts:
Long ago, it was theorized that light had to travel through a medium, until the particle/wave duality of light was discovered.  Then it changed to the idea that light can travel through the vacuum of space as a particle.

Light travels at different speeds depending on the medium that the light is traveling through.  It travels slower in water than it does through air.

However, is there any possibility that dark matter is a medium that light travels through?  Could it be conceivable that what we recognize as light speed is really just the speed at which light travels through this medium?  If we were to remove dark matter, would light go faster?


texts: Why are the high density regions in outer space black holes?

texts:
The escape velocity of an object is given by the following formula.

$$v_e=\sqrt{\frac{2GM}{r}}=\sqrt{\frac{2 \mu}{r}}= \sqrt{2gr}$$

As far as I am aware, the mass of the star which formed the black hole is the same as the black hole itself, the only difference is the black hole's density.

Now, the escape velocity of the star which formed the black hole is not as high because light can escape the star. Considering the equation above, $G$ is constant, $M$ is constant, however, $r$ changes resulting in a much smaller region as compared to the star. Consequently, it's the radius which determines the high escape velocity of a black hole, not its density. Then why is it that I always hear that black holes behave the way they do because they are highly dense regions in spacetime? What am I getting wrong? Thanks.


texts: What is the minimum mass of a celestial object so that it can have a moon?

texts:
I was wondering how massive something have to be so that it it can attract moons by pop culture standards (ellipsoid/round shape).
Could a planetoid have a moon? What is the relation between the mass of a moon and planet?


texts:
I really want to take pictures of Starlink satellites, but I am fairly new to this kind of photography so I have a few questions. I am using a Nikon d3400 and a tripod obviously.

What is the best lens to use and do I need any additional filters?
How do I point my camera at the spot where the satellite will pass through, do I need some kind of tracker for it?

Thank you in advance for your answers, I hope my questions made sense.


texts: I need help getting mars to look like a circle

texts: I am having a lot of trouble getting a sharp image in my telescope. It is a skywatcher with a focal length of 1200mm and a diameter of 254mm. I have a laser colimater that I got for Christmas, so I have been using that but when I try to look at mars I see three mars in an overlapping triangle. I am not sure what to do. I have made sure that the laser is straight by 1. spinning it in the eyepiece holder and the laser stayed in the same spot 2. zooming in and out and again the laser stayed in the same place. the laser seems to be centered so I put it in the middle of the doughnut on the mirror and the laser bounces back into the middle of the colimating lens, but when I look at mars there is still three and there is no detail its just a red fuzz ball. I also checked the collimation of the telescope by zooming all the  way out to see where the shadow was and it seemed to be right in the middle.hopefully this makes sense, but let me know if I should say something in a different way. thank you In advance.


texts:
What methods exist to calculate the ellipticity of galaxies and what are their drawbacks? I have asked this question about ellipticity in the SDSS but I want to know about general methods for cases where I have just an image of a galaxy.

I do so far know these methods:

ellipticity from stoke parameters or the flux-weighted second moment as described here.
from adaptive moments as described here (which would allow me to do shear calibration).
fit an ellipse on a particular isophote as decribed here.

My question is whether there are further methods I am not aware of. Does there also exist a method when I fit an ellipse to all isophotes? And how would I calculate the 'average' from it (such that the spherical center does not contribute to much)?

I would like to learn about the pro and cons of each method. I am especially interessted in methods that get close to what people would estimate by eye. I would also be glad to hear about references (textbooks or papers) for the different methods.


texts: Is there a spectrum of polarization? Does polarization vary depending on wavelength, from one and the same source?

texts: Is it possible, and does it occur naturally, that the light from the same source is simultaneously polarized differently at different wavelengths? Light can be polarized by interstellar magnetic fields. Could it polarize light of different wavelengths differently? Does a pair of polarized sunglasses polarize radio waves and UV in the same way as visible light?


texts: How can we tell that a short-period binary is tidally locked?

texts:
Observationally, how can this be established?

What is known theoretically about the process?

Are there any reference papers to read?


texts: Syncing time for measuring the parallax of Mars

texts: I read somewhere (I can't remember the website) that when Cassini and Jean Richer measured the parallax of Mars, they used Jupiter's (or maybe Saturn's) moons to sync their time. How was this done?


texts: Is there a way to prevent chromatic aberration without using an achromatic lens in refracting telescopes or in any other instruments...?


texts:
I live in Hyderabad. Around a week ago, I, with my mother went to observe the planets from my roof I had observed Jupiter and Saturn till they went down below the horizon and then I was ready to move my telescope on Mars which was about just above my head. When I looked at it I saw a really strange 5 dotted flying thing fly a little towards the left of Mars. I asked my mother, and as she had sight and was not wearing spectacles, She didn't see it as 5 dots, but a thin straight line.
I know that it cannot be a rocket or jet plane as it left no trail, and it can't be  a plane or helicopter as it made no sound. I read a similar post about 2-3 weeks ago about a U.F.O collecting stars, but this was not at all like it. Could anyone tell me if it was anything known or if that is really an Undefined Flying Object?


texts: Do changes in the orbit of the Earth contribute to global warming

texts:
"Researchers have largely put to rest a long debate on the underlying mechanism that has caused periodic ice ages on Earth for the past 2.5 million years – they are ultimately linked to slight shifts in solar radiation caused by predictable changes in Earth's rotation and axis."

How do we know that the amount of solar irradiation has increased since the ice age. Do current changes in the orbit path still cause the average temperature to still be increasing until another such event causes another orbit change?

This appears to be the article.


texts: What is the Lyman Alpha forest Used For?

texts:
The Lyman-alpha forest is an interesting feature found in the spectra of sufficiently distant objects. This series of absorption lines extends over a range of frequencies, and are a result of the Lyman-alpha electron transition of neutral hydrogen in between the source and the observer.

My question is:
What information can be gleaned about the universe from this spectral feature?  In other words, what areas of research use the Lyman-alpha forest as a tool, instead of seeing it as noise?


texts:
On this link, it states the following: "large assemblies of galaxies that are permeated by even larger amounts of diffuse gas. With temperatures of 10 million degrees or more".

How are these diffused (ionized) gas able to become so hot when they are in large distances from one another and have very little density?


texts: Catalogs for type K stars and their planetary systems?

texts:
I am doing data collection on all type K stars, and their planetary system so I may develop an algorithm for a planetary formation simulation with the Unity game engine.
Where could such a catalog of stars be found?


texts: How to collect photometric data for WW Aur

texts:
I have a problem with the collection of photometric data for star WW Aur. I can't find any file with HJD and mag column beside this one on Vizier.

I need more than one source of data. I will use the data to create a light curve via Scidavis software.


texts: Speed of light through the ISM and Wavelength

texts:
Articles written about fast radio bursts describe the signal's short-wavelength components arriving before its longer-wavelength components, suggesting energy-dependent time delays in the interstellar medium (ex. http://www.scientificamerican.com/article/a-brilliant-flash-then-nothing-new-fast-radio-bursts-mystify-astronomers/).

Why are lower-energy signals slowed slightly more than higher-energy signals when passing through clouds of electrons? Also, how do we know about this slowing effect if it appears to be so small (what objects or events have exhibited the time difference)?

My first-order logic guessed that higher energy radiation is slowed more, remembering the mnemonic "Blue bends best" from optics. I assume these intergalactic clouds behave quite differently from a solid prism.


texts: Putting mass-luminosity relation and Hertzsprung-Russel diagrams together leads us to a mass-age relation; so how do stars lose their mass over time?

texts:
I think that the title is completely clear, but here's an expansion:

I was just reading about Mass-luminosity relation that says massive stars are more luminous than tiny ones. Well, let's talk about main-sequence stars for now. This relation becomes interesting when it's mixed with the Hertzsprung-Russel diagram that says young stars are more luminous than older ones (Originally says that hotter stars are more luminous and we know that hotter stars are younger).

So mixing these two relations produces some kind of Mass-age relation which says young stars are more massive, that is a star loses its mass over time, right? (Please tell me if I'm going wrong)

If this Mass-age relation is right, how do stars lose this extra mass during their evolution? And where does this stray matter go?


texts: Low precision GMST formula clarification

texts:
According to the website http://aa.usno.navy.mil/faq/docs/GAST.php, a low-precision way of calculating the GMST is via the following formula:

GMST = 18.697374558 + 24.06570982441908 D

The website doesn't actually state where the numbers come from. Another website tries to do this:

"To compute of sidereal time with low precision use of the low precision formula: GMST = 18.697374558 + 24.06570982441908 D, where 18.697… is sidereal time of the reference time 2000-01-01 at 0 UT, 24.0657… is a ratio of synodic and sidereal periods of Earth and D is days (and its fractions) since the reference time."

but I don't understand what this 'ratio of synodic and sidereal periods' means. I know what the sidereal and synodic days are and their values, but how do we use those to end up with the number 24.0657...? If anyone could help me out that'd be much appreciated!


texts: What reference frame do age-of-the-universe calculations assume?

texts: I'm thinking, in particular, about general relativity. When we speak, for example, of neutrino decoupling, what do we mean when we say this happened in the first second after the Big Bang? Do we mean one second from the point of view of an imaginary observer subject to the same relativistic effects on time? How much can we even say about the affect of relativity in this context?


texts: How many streetlights should we turn off to decrease light on the ground by about 2.512 times?

texts:
I am looking for some guidance for a back-of-the-envelope-calculation to show that turning off street lights can make a difference for light pollution in the style of a Fermi problem.
My first step is to abstract all urban area as a concrete or asphalt patch with a grid of streetlamps on it.

The illuminance on the ground is probably 10 lux.
Street lamps are of the order of 10m appart (1m would be too close, 100m too far). This is not necessary for the light balance, but would later allow to estimate how many street lamps we have, and how much energy we could save.
The reflectivity of concrete is around 0.2-0.3, see arch.ethz.ch
Google claims that 3% of the Earth's area is urban area. Let us be optimistic and assume everything else is pitch black at night. Let us further simplify and say that at each point in time, 1.5% of the Earth dark side is emitting light.
For simplicity, I would assume all urban area on sea level.
Finally, let us assume for the moment that there are no clouds above our huge hypothetic shining concrete parking lot.

But how do I proceed further? Is there some easy formula to relate light emitted on from the surface into the atmosphere with the magnitude of stars one could observe?
Edit 1
First we should calculate which magnitude one would see on average with the assumptions above and see whether that makes sense. I hope/expect the answer would be something like magnitude 3 or 4 with light pollution.
Background
Since reading the German website "Godfathers of the night" (Machine translation) I keep thinking how to make the idea of turning off street lamps (at least partially when nobody is there) more attractive for everybody, not just for sky or animal lovers. To convince people, numbers usually help.
References

European Guideline EN 13201-1-5 - Road lighting

How can I make Stellarium match the magnitude of stars visible in London, UK?


texts: Is there a term for asteroseismology as applied to giant planets?

texts: Giant planets such as Jupiter have oscillations which enable analyses using the techniques of asteroseismology, for example Gaulme et al. (2011) detected global modes on Jupiter via radial velocity measurements. Is there a specific term for this technique as applied to giant planets instead of stars?


texts:
I encountered this statement that the dark side of the moon is hidden from our sights.
Could someone please explain?


texts: Where is a Hollow Object's Center of Gravity?

texts:
Let's imagine that the Earth's moon is hollow and the remaining mass is structurally strong enough to retain its spherical shape (like a really big tennis ball).

Given this scenario, where would its center of gravity be? What would the conditions be like inside? Also would one still be able to stand on the moon?


texts: Can the explanation of Tabby's star KIC 8462852 be an intervening planet in orbit around another star?

texts:
Can an intervening planet in orbit around a nearby star cause periodic occultation?

Please excuse the crude text graphics:

Tabby ...... PlanetP ......................................... Earth
                |
             StarX

StarX and Tabby's star are close to each other (maybe part of some local cluster, like the Centauri brothers) so the line of sight with our Sun/Earth has maintained over the recent handful of years.
PlanetP orbits StarX with a period of around 750 Earth days.  Its ecliptic is at an angle (somewhat perpendicular) to the line of sight with Earth.
PlanetP's orbit rotates (precession?) a bit around StarX, so the amount of occultation of Tabby's star by PlanetP varies from 15% to 22% to whatever.


texts:
I am curious to know If dust extinction were to vary as a power-law so that $A_{\lambda}$ proportionate to $\lambda^{\beta}$ , what value of beta
would give rise to Rv =3.1 which is the average in diffuse ISM. I can see a plot but I wanted to see how to derive Rv=3.1 mathematically


texts: Star like light moving in the sky, what could it be?

texts:
I was camping this weekend away from the city, so my friends and I decided to spend one of the nights watching the sky, since it's impossible to see anything around where we live. After a little while, of staring up, I saw what looked like a star moving. It moved fast, but not a long distance, and various directions and back. It was about the same size of the other stars near by. It was not as bright as other stars though. I stopped looking up for a little bit, to make sure my eyes were seeing things correctly. But then I looked up again I continued to see the light moving. I thought I was going crazy, so I pointed it out to my friend. It took him a little while, but after a few minutes he also saw it. His girlfriend also saw it a few minutes later. My friend brought up something about satellite reflections and other theories, but we have no idea what we saw.

So what could it be?


texts:
I remember reading an old astronomy book mentioning a certain planet and the main point of interest was that this planet has or may have polar ice caps but of acid. I can't remember the name though, what planet or planets fit this description?

Now there some very strange worlds out there though I'm unsure whether this one may or may not be all that strange. Is this something of an oddity or is a planet containing frozen acid not that uncommon?


texts: How can we know the nature of the companion in a neutron star binary?

texts:
The companion of a neutron star could be a main sequence star, white dwarf, neutron star or black hole.

We detected a pulsar-pulsar binary several decades ago. But if one of the pulsar does not radiate towards us, we may be not able to tell it is a pulsar or a black hole. If the companion is not a main sequence star, how do we know the nature of the companion in a neutron star binary?

Of the binary systems with one single neutron star, is that possible there is another neutron star or a hidden black hole?


texts: The universe is dying vs the universe is ever expanding

texts: How does the idea that the universe is dying (trumpeted as some big new revelation the past few days on several new sites I keep seeing pop up on google news) square with the idea that the universe is ever expanding?


texts: Can I see the star of Bethlehem at Christmas?

texts: In the Christian tradition the first Christians are believed to be three wise men who followed the star of Bethlehem to the stable where Jesus was supposed to be born. Is it with some accuracy known if this is an actual star and if so will I be able to see that star during Christmas in North-western Europe?


texts: Derivation of the formula for longitude of ascending node for a satellite

texts:
I've been looking into the document IS-GPS-200H to understand how to calculate satellite location in the ECEF coordinate.

I am having problem understanding the formula to derive $\Omega$, the longitude of the ascending node (LAN) relative to Greenwich at given time $t$:

$$
\Omega = \Omega_0 + \left( \dot{\Omega - w} \right)\times t_k - w \times t_{oe}
$$

where:
$$
\Omega_0: \text{LAN relative to vernal equinox, at the beginning of the week}\\
\dot{\Omega}: \text{angular velocity for LAN, relative to vernal equinox.}\\
w: \text{angular velocity of earth, relative to vernal equinox.}\\
t_k: t - t_{oe}\\
t_{oe}: \text{ephemeris reference epoch}\\
$$
(and let us denote the beginning of the week as $t_0$ for brevity).

But if I try to work out this from scratch:

At $t = t_0$, LAN was $\Omega_0$. But since what we really need is the difference of LAN and longitude of Greenwich, we also need to know $w_0$, the initial longitude of Greenwich at $t = t_0$.
$$
\Omega(t = t_0) = \Omega_0 - w_0
$$
At the ephemeris reference epoch time $t = t_{oe}$, LAN and the earth both rotate with their respective angular momentum and hence:
$$
\Omega(t = toe) = \Omega_0 + w_0 + (\dot{\Omega} - w) \times t_{oe}
$$
As time varies from $toe$ to $t$, again LAN and the earth both rotate with their own respective angular momentum and hence
$$
\Omega(t) = \Omega_0 + w_0 + (\dot{\Omega} - w) \times t_{oe} + (\dot{\Omega} - w) \times t_k
$$
which obviously differs from the right formula by $w_0 + \dot{\Omega} \times t_{oe}$.

My question is where am I making mistakes/misunderstanding the eqution?
Explain also why we don't need to know $w_0$ or equivalent input, that would be greatly appreciated.


texts: Size and mass of comet ATLAS (C/2019 Y4)

texts: I read some news about this new comet and some unreliable sources said it is twice as big as Jupiter. I tried to find a reliable estimation of its size or mass, but I couldn't find anything. Would you please provide me a general way to distinguish reliable facts from unreliable facts about comets? Like their size and mass range, how much mass they lose, etc. Thank you in advance.


texts:
In the science fiction book 2010, monoliths increase the mass of Jupiter until it becomes a star.

https://en.wikipedia.org/wiki/2010:_Odyssey_Two

  The Leonov crew flees Jupiter as a mysterious dark spot appears on
  Jupiter and begins to grow. HAL's telescope observations reveal that
  the "Great Black Spot" is, in fact, a vast population of monoliths,
  increasing at an exponential rate, which appear to be eating the
  planet. By acting as self-replicating machines, these monoliths
  increase Jupiter's density until the planet achieves nuclear fusion,
  becoming a small star.

Obviously fiction.  But my question is about orbits and the mass of orbiting bodies.  For planets, whose mass is negligible compared to their star, I understand one can ignore mass for purposes of calculating orbits.

But if Jupiter were by scifi magic made as massive as a star, would its orbit remain the same regardless?  Or if it were made to stay on its same orbital path, would it have to move more quickly, completing its orbit of Sol in a shorter time?


texts:
I want to make clock that shows current time on different solar bodies. For that I need to know how to count time on those planets. Is there any official definition of times on other planets in our solar system? Brief google search did not help.

I am capable of implementing library for this myself but I wonder about current state of the art of time telling on other planets. So basically I want to know these things:

Is there any official time standard?
If so, where can I find it?
If not, how should I approach this problem? Should I make something like Unix epoch at some time in the past for every planet?

Thanks for any tips.

EDIT: Ok, so on Mars, seconds, minutes and hours were made longer to keep 24-hour day. Is this viable to other planets as well? Venus aside for now :)

I was also thinking about keeping the hours the same, having days as long as many whole hours fits in them plus smaller compensation hours which would have length of (total_minutes_in_solar_day % 60). This concept is from David Weber novels, if you are familiar with them.

EDIT2: Based on discussion in the comments, I'll try to make some basic model that would be if possible applicable to any planetary body (thinking ahead :)). This is obviously not going to have any serious real world application but I would still like it to have some sense.

Thanks for your input, I will leave the question open for some more time if anyone wants to add their opinion.


texts: Can you view the Perseid meteor shower anywhere on Earth?

texts:
The peak viewing dates of the 2016 Perseid Meteor Shower is said to be between the dates of August 11 through the early morning hours of August 12.

Being a resident of the United States, I keep getting my peak viewing dates and times for my area and for the whole of the United States.  There is never any mention of any other parts of the world having the capability to view this meteor shower.

Is the Perseid Meteor Shower visible to all parts of the world?


texts: If Mercury and Venus where to collide would it make a habitable planet?

texts:
I have been wondering for a while now about a scenario of where if you combined Venus & Mercury would it be habitable if located in the goldy locks zone? I have found in my little research that Venus doesn't have an internally generated magnetic field. But Mercury does indeed have an internally generated magnetic field. Though weaker than Earths of course.

I don't really know what other factors for making it to whether or not this new plant would be able to sustain life.

So my question is, would the new planet be habitable, or are there more factors needed to determine this?


texts: Can a star emit a larger fraction of its total light in the visual range than the Sun does

texts:
If one sets both the bolometric and visual luminosity of the Sun to 1 (and use a linear scale), can a star then have a higher visual than bolometric luminosity?

It makes sense that this will not be the case for most stars, as smaller stars will tend to radiate more of their energy as infrared, while larger go more into the ultraviolet. That is, this is inherently linked to what wavelengths the human eye has evolved sensitivity for. But are there exceptions? If there are, I would think they have parameters very close to the sun.


texts:
Stars plotted by luminosity and surface temperature fit into patterns in a Hertzsprung–Russell diagram.  A rough diagonal subset of this plot is called the main sequence.  Is this in any sense a temporal sequence?  There's a clue in the stellar physics section of the wikipedia article that the answer is no, but that it once was thought so:

  Contemplation of the diagram led astronomers to speculate that it might demonstrate stellar evolution, the main suggestion being that stars collapsed from red giants to dwarf stars, then moving down along the line of the main sequence in the course of their lifetimes.

So does the word "sequence" in this case now mean just a particular ordering, and not a progression over time that any one star makes?  Is the main sequence just some kind of plateau in stellar evolution where stars spend a significant duration?


texts:
The paper "M dwarfs: planet formation and long term evolution" describes blue dwarf stars, a hypothetical next-stage in the lifespan of red dwarf stars within a certain mass range, after which they become white dwarves.

(Hypothetical because the universe is not old enough for any red dwarves to have reached this stage.)

The Hertzsprung Russell diagram on page 917 of this paper, if I've interpreted it correctly (I'd never seen one of these until recently!), shows red dwarves in the 0.14-0.16 solar mass range progressing further away from the "red" end of the spectrum and in the direction of the blue. But it does not actually depict their surface temperature reaching - or indeed exeeding - 9000K (spectral type A, instead of the B or O-type that are normally considered blue enough to justify the name.)

Am I correct that "blue dwarf" stars would not, according to this paper's H-R diagram, actually ever get past "white", and are thus named because they've moved a long way from the red end of the spectrum toward the blue? I have seen different colour charts depict different levels of blueness for the O, B and A types while researching this question, although the majority consensus seems to be that A types are almost entirely white with only a very small amount of blue.

(The same Hertzsprung-Russell diagram is also shown on page 424 of the 1997 paper "The End of the Main Sequence", if that helps at all.)


texts: How is the distance from Earth to distant celestial bodies calculated?

texts:
It's difficult for most people to grasp how distances between the Earth and distant celestial bodies are calculated. For example, when someone tells me that a given star is 100 light years away, how would I determine if that were accurate?

I think that my question boils down to three general use cases:

How can I calculate the distance between Earth and a local body in our solar system such as the sun, Jupiter, our moon, or Neptune?
How can I calculate the distance between Earth and the nearest star outside of our own solar system, Alpha Centauri?
How can I calculate the distance between Earth and an extremely distant object, such as a star 100-1,000 light-years away?

I've heard of using red shift to calculate large distances, but what exactly is this and how does it work?


texts: Are there any examples where the transverse doppler effect is applied in astronomy?

texts: Are there any astronomical examples where the transverse doppler effect(Horizontal doppler effect) is applied (Derives a meaningful result)?


texts: Is the composition of stars in future made of more and more heavy elements?

texts:
In the beginning stars only consisted of the hydrogen element and due to nuclear fusion of those elements in stars and supernova's more heavy elements were created. Because of that, like our Sun, the stars were made not only of hydrogen but also oxygen and carbon etc.

Now is it expected that in future more stars will be made of more heavy elements or are there causes/laws which prohibit stars forming of e.g. stars made of elements without hydrogen.


texts: There are various good models for solid-body tides on the Earth. Are there any recommendations for tides on the Moon?


texts: What telescope might this be a model of, with Sir Fred Hoyle?

texts:
The BBC article Archaeopteryx: The day the fossil feathers flew has nothing to do with Astronomy but it shows a file photo of the famous astronomer Sir Fred Hoyle with a model of what looks like some kind of telescope.

I don't know if the "dish" is supposed to be fixed metal to reflect radio waves, or a rotating dish of mercury to reflect light, but I'm guessing the former is more likely. It looks like an off-axis, tilted secondary mirror reflects radiation back to ground level, but it's just a model so I can't be sure.

Are there any ideas what real instrument (if any) or at least what design that this fixed-"dish" model might be a model of?

What's scary is that at the bottom right it looks like a building several stories tall, which means that the telescope is huge, and the size of Sir Hoyle is on par with Godzilla. The time frame seems right for that too! ;-)


texts: Do we know what's "behind the Milky Way"?

texts:
If we do, how? And what are those objects on the other side of the galactic center?
We can't see through the dense band of light and dust, but maybe in other spectrums the band is much less dense? But I doubt it.
Maybe we could detect or have detected those galaxies/satellites through their center's strong X-Ray? Or through their gravitational force on the Milky Way?
If not, then there is a chance, that there is a closer galaxy than Andromeda and even more satellite galaxies than currently observed?
Thanks in advance! :)


texts: How does the Sun generate its periodically reversing large-scale magnetic field?


texts: What is the minimum mass required so that objects become spherical due to its own gravity?

texts: Is this minimum mass known? or maybe, is it given in terms of density? If so, how much density is the minimum to have an spherical object due to its own gravity?


texts: What would we see if all stars disappeared simultaneously?

texts: If all the stars disappeared simultaneously (from the earth's reference frame), how long would it take for the night sky to go completely dark to our unaided eyes? Would we still be able to see anything after a thousand years?


texts:
Are there examples of single stars that are a part of multiple constellations? What about stars that are part of one Western constellation and one non-Western?

Thanks in advance!


texts: How can I calculate how long time a Galilean moon orbital configuration will take to repeat again?

texts: For illustrative purposes, if I assume that the Galilean moons, Io, Europa, Ganymede, and Callisto, have coplanar circular orbits, and their initial configuration are, respectively: 10, 20, 30, and 40 degrees, measured from an arbitrary axis, when this configuration will repeat again? Considering that the four moons have their actual periods.


texts: Universe is expanding, why doesn't time expand with it?

texts:
Reading this response When the universe expands does it create new space, matter, or something else? I understand Universe is not expanding like if it is stretching its space, but it is actually adding space that matter can travel through.

So, if the Universe can create more space, and space and time are related, shouldn't that mean Universe is creating also more time? And if it is the case, shouldn't this effect alter results of our measurements about how much time light spends to reach us from distant stars?

We know that photons have no mass, travel at speed of light, and time has no sense for them. But at the same time they are influenced in their trajectories by folds in the space-time.

If gravity deforms space-time, shouldn't an expanding Universe adding new space doing the same?


texts: Deformable optics for consumer telescopes

texts: I am aware that there are adaptive imaging techniques for amateur astronomers. But these methods correct an image on a screen. This is nice, but to me the real lure of amateur astronomy is the act of seeing an object through the eyepiece, not on some computer screen. Are there any deformable-mirror technologies available for consumer-grade telescopes that would allow for direct viewing of the object with correction?


texts: Is there any difference between rotation axis 90 degrees and 270 degrees?

texts: Is there meaningful reason to distinguish 90 and 270 degree rotational axis?


texts: I'm not referring to a star made of dark matter or a Newtonian dark star. I'm talking about a star powered by dark matter annihilation. I imagine it would form close to the centers of galaxies where there is a large amount of dark matter.


texts: How to fix a Direction in the context of universe?

texts: My question is regarding the direction/orientation of earth. Is it a constant with reference to universal parameters?


texts: Just a very general question, but I haven't seen this discussed elsewhere. If a moon gets busted up for some reason, say a collision with another moon, and is at the right orbital distance from its planet, how soon would you expect the debris cloud to be recognizable as a ring? I'm just looking for a ballpark answer: 10 thousand years, a million years, ten million, etc.


texts: Kinetic energy and galaxy arm rotation rate

texts:
The higher than expected rotational velocity of stars and gas clouds in the outskirts of galaxies is explained today by invoking dark mater that supplies not only the additional gravitational mass, but explains the amount of gravitational lensing of more distant galaxies.  Furthermore, dark matter distribution about spiral galaxies places it on the outside of galaxies and not so much on the inside.
Certainly the higher than expected velocity on the outside of galaxies also translates into higher than expected kinetic energy. Should the extra kinetic also increase the gravitational stress energy tensor in that region of space.
If so, do our models already factor in kinetic energy and it's gravitational stress energy tensor,  overlook it, or the effect is just too minuscule to be of any importance?
I don't expect the extra kinetic energy to be a replacement for dark matter, and suspect the effect may be too small to be of much importance, but the kinetic energy distribution  around a galaxy, intuitively on the surface, appears to have the right distribution, so I'm asking generically, do astrophysics factor this in their models, or not, and if not, should it?


texts: For how many bodies can there be a stable orbit with no very heavy central body?

texts:
The question is a bit vague, but let me explain:

Take for example 2 bodies of the same mass. They can orbit around their centre of mass/gravity. Is something like this possible with multiple bodies of varying mass and distance from the centre of gravity without a relatively extremely massive body there?
i.e. would it be possible to have a planetary system that orbited around the centre of mass without there being a massive star (roughly) in the middle?

I know that the Sun also has an orbit around the centre of gravity of the solar system, but that orbit is very small compared to its size, which puts it roughly in the centre.


texts: What happens when a huge star is being consumed by a black hole

texts: What happens when a huge star in its later stage( iron core) is near a black hole that slowly consumes some of the matter from the star? Would this offset the imbalance caused by iron fusion prolonging/ deterring a supernova?


texts: Question about $3p/c^2$ in Friedmann equation

texts:
The Friedmann Equation:

$$\frac{\ddot{a}}{a} = -\frac{4\pi G}{3}\left(\rho + \frac{3p}{c^2}\right) + \frac{\Lambda c^2}{3}$$

I want to know, where does the $3p/c^2$ come from. I know it is something like density of energy or something like that but I don't know where it comes from.


texts:
So there is this huge picture by Hubble.

Better yet, with the zoom tool

And if you zoom to the maximum level, you can see a seemingly uniformly distributed dots of orange and blue. Are those actually stars and maybe galaxies behind them and not just camera noise?

Furthermore I'm trying to get a grasp on how large portion of the sky this photo represents.

  At 3.4, the apparent magnitude of the Andromeda Galaxy is one of the brightest of any Messier objects, making it visible to the naked eye on moonless nights even when viewed from areas with moderate light pollution. Although it appears more than six times as wide as the full Moon when photographed through a larger telescope, only the brighter central region is visible to the naked eye or when viewed using binoculars or a small telescope.

Let's say this is about the half of the diameter of the galaxy so this photo is about ~3 Moons wide, is that correct?


texts: Let's say you had a binary planet system of two earth-sized worlds with about the same mass. Could a stable moon orbit around the barycenter of both planets? If so, is there a limit to the mass of such a moon? Would it have to orbit in a Lagrangian point?


texts:
Can anyone explain why the Moon looks so small and dark in photos taken from cislunar space during the Apollo 12 mission? AS12-53-7887 found at https://www.lpi.usra.edu/resources/apollo/frame/?AS12-53-7887

The image was taken with the 70mm Hasselblad and 80mm lens using a fast film. The photo index can be found here:
https://www.hq.nasa.gov/alsj/a12/a12.photidx.pdf


texts: On a Hertzsprung-Russell diagram, why is the main-sequence line squiggly?

texts: When I look at a Hertzsprung-Russell diagram, I'm able to see a main-sequence line from the most luminous main-sequence stars at the top left to the dimmest main-sequence stars at the bottom right. However, this line isn't straight - it's quite squiggly. Why is the line squiggly, and not defined by a polynomial or rational function?


texts: What is the highest redshift a galaxy with a known stellar mass can be observed?

texts: As part of my research, I would need the following information which it seems I cannot find it in literature. I would like to know what are the highest redshifts at which $10^{7}$, $10^{8}$, and $10^{9}$ $[Solar Unit]$ galaxies can be observed let's say by current HST?


texts: How is the total mass of all the Solar System's small asteroids and meteoroids estimated?

texts:
This diagram from wikipedia (shown below) graphs asteroid Diameter against Number of asteroids, for the Solar System.  (Graphic is courtesy of the Marco Colombo, DensityDesign Research Lab).

Presumably the Diameter data for the larger asteroids is strongly supported by observations.  But for progressively smaller bodies direct observational data is presumably less available.

The pattern shows a strong, but not perfect, linear inverse correlation between log(Diameter ) and log(Number).  This seems quite reasonable for large bodies. It also sounds plausible that such a pattern continues into the realm of small asteroids and meteoroids.

But what is the physical justification for extending the log:log pattern into the realm of small bodies which are too small to see from Earth?  Presumably the current estimate of the total mass of the Astroid Belt (4% of the mass of the Moon) is consistent with this distribution model.  But can we confidently rule out the possibility that the linear log:log pattern breaks down for small objects allowing, for example, a much larger number (and mass) of small bodies to be present in the Asteroid Belt, sufficient say for the total combined mass to equal 100% of our Moon's mass?


texts: What do the acronyms in "AT 2017 gfo" mean?

texts:
About 11 hours after the detection of GW170817 by Ligo/Virgo and the quickly succeeding detected GRB170817, a transient afterglow in the optical-nIR frequency appeared which showed many signs of r-process nucleosynthesis. This transient was given the identification AT 2017 gfo.

  "SSS17a, now with the IAU identification of AT 2017 gfo"

My questions are simple. What do the acronyms "AT" and "gfo" mean? And for that matter, what does the acronymn "SSS" stand for? I tried to do a quick search through the IAU website but all I could find were general instructions on how to come up with your own astronomical identifier. I suspect "AT" means astronomical transient, but I can not confirm.


texts: Potential formation mechanisms of the interstellar asteroid 'Oumuamua

texts:
NASA recently discovered 'Oumuamua, an interstellar asteroid that appears to have a very elongated shape, unseen in solar system asteroids.

What physical mechanisms, in the progenitor stellar system or from interstellar transit, could lead to the formation of an asteroid with such a shape?


texts: Can a black hole undergo a Penrose process inside another black hole's ergosphere?

texts:
I recently watched this video on black holes:
https://www.youtube.com/watch?v=ulCdoCfw-bY

And that statement should give you an idea of my level of understanding :)

I was wondering if a spinning black hole A could complete a Penrose process through another spinning black hole B's ergosphere. i.e. if it could travel through the ergosphere of another spinning black hole and emerge out of it the other side.  What would happen?  Perhaps this is related to the question of whether black holes can orbit each other (in two or three-body system)?  What happens if black holes orbit each other, but their ergospheres overlap?


texts: Which of the planets would be detected if they were exoplanets?

texts:
An (almost-)omnipotent being selects the solar system, presses Ctrl+C, and then Ctrl+V several times, creating copies at distances of 5, 500, 50.000, and 5.000.000 light-years away. All in a direction where the instruments we have are currently directed that way.

So we see these four star systems at various distances.

With our current technology, of 2020, which of the planets will we discover in each of these distances?


texts: on/off filter and their photometric calibration

texts:
I am reading a paper here

What is off band filter for? If we have already known the positions of the sources we are interested in, then off band filter is useless, correct? So an off band filter is to discovery sources?
An off band filter is usually self-designed? There is no unified standard(central wavelength, band width, etc. )?
For the photometric calibration, when is continuum needed? Since there are standard stars, why is continuum needed?

Thank you. My questions are long, so I put them here.
We assume the continuum is flat, right? The off-band filter is for 1) photometric calibration, 2) discovering new sources?
How to do the photometry then? When do we use continuum and when do we use a standard star?


texts: How does SOHO know what's going on in the sun's magnetic feild?

texts:
SOHO can image the sun's magnetosphere. I don't have the foggiest idea how they manage it, because at most a satellite can have a single compass, and measure one point in the magnetic field. I thought for a while that maybe they were using the Zeeman effect, but their image description says that "black and white indicat[e] opposite polarities." I don't think you could detect the direction of the field with Zeeman splitting, because the thermodynamic distributions should be isotropic in every variable, and there's nothing on the atom scale to compare the direction of the field against.
How does SOHO manage to image the magnetic field of the sun?
https://sohowww.nascom.nasa.gov/data/realtime/image-description.html


texts:
Writing about the 2018 occurrence of the annual Perseids meteor shower, the EarthSky.org Astronomy Essentials article Start watching for Perseids now says:

  People tend to focus on the peak mornings of the shower and that’s entirely appropriate. But meteors in annual showers – which come from streams of debris left behind in space by comets – typically last weeks, not days. Perseid meteors have been streaking across our skies since around July 17. We’ll see Perseids for 10 days or so after the peak mornings on August 11, 12 and 13. What’s more, the Perseids tend to build up gradually, yet fall off rapidly. So it’s possible to catch Perseid meteors now.

Looking at images in @userTLK's excellent answer to "What (actually) is Jupiter doing to this year's Perseids meteor shower?" one can imagine the Earth passing through a "fuzzy tube" around the orbit of the shower's associated comet's orbit, which is often inclined with respect to the ecliptic.

So far I can't imagine a mechanism that would make the ramp-up and then ramp-down happen at very different rates. Why would the Perseids meteor rate fall off after maximum faster than the increase before maximum?

I've just found this artist's conception of the "fuzzy tube" of debris in Sky and Telescopes's “Great Show” Predicted for Perseid Meteor Peak on August 12–13.

above: "Every year, in mid-August, Earth passes collides with particles spread along the orbit of Comet Swift-Tuttle." Credit Sky & Telescope, from here.


texts:
Here is a good example picture of what I mean:

If not, why is this so? I've heard often that in the creation of a planet more the rings have formed, they are most likely just all over the place before they settle to the formation that we know, so why not come to a stable position in such a way. Note that the picture is only an example, rings that horizontal to each other are included too.


texts: What happens if the Moon moves twice as fast?

texts:
If the Moon would rotate around the Earth with double its current speed.
What would happen then?
Would it get further from the Earth or it would be possible to keep its distance from Earth?


texts: Is it possible to break apart a neutron star?

texts:
I was inspired by this question on Physics, as well as this question right here on Astronomy. Neutron stars are tightly bound together as neutron degenerate matter. They're very massive and have a strong gravitational field. Is it possible to break one apart into sizable chunks? How would you do this?

The answers given are good and answer my question; I'll just clarify some one thing (based on the comments) for posterity.

I would define "broken" as when any significant amount of mass is removed from the neutron star, as in mass shedding, as Mitch Goshorn wrote. The resulting object, however, should contain a significant amount of neutron matter - that is, it should largely retain its prior composition.


texts: How to calculate parallactic angle from a fixed alt-az position?

texts:
Say we have an observer at a specified latitude and longitude on Earth who would like to observer a specific fixed alt-az sky position (rather than RA and dec), for example to look at geosynchronous satellites.  Since the alt-az position of the observation is not changing, it seems to me that parallactic angle also would not be changing and so should be derivable from just alt-az and lat-long, but I haven't been able to figure out how to do it.

All the calculations I've seen online use RA or hour angles in calculating parallactic angle, which intermediary values I would like to avoid if mathematically possible.


texts: Which is really larger, Big Dipper or Small Dipper, in 3D

texts:
If we take the convex hull of the 3D coordinates of the stars in Ursa Major and Ursa Minor: which hull has larger volume?

This is meant semi-humorous but we might also learn some interesting points from good answers, e.g. which metric to use when mass distorts space or how to measure with respect to frames of reference.

Update: to those asking about which stars to consider, just choose e.g. the canonical 7 stars of each. But this question is less about one arbitrary number and more about which things to consider, e.g. to show beginners that a bright star is not always close (and the other way around), and that size can be different in 3d than our arbitrary 2d-perspective.


texts: Can someone calculate the age of the KOI-4878 star?

texts:
The star’s low metallicity and fairly high space velocity suggest that KOI-4878 is older than the Sun.

But I don't know how to calculate an estimation for the age.

KOI-4878 data on Simbad


texts: How can I calculate the uncertainties in magnitude like the CDS does?

texts:
When you check an entry of the Gaia DR2 stellar catalogue in Vizier, for example this one, you can see that there is a value for the G magnitude (in our case 18.0733 mag) and a value for the uncertainty on that (in our case 0.0023 mag). The thing is that the G value is part of Gaia DR2 but the uncertainty is not, it was calculated by the CDS for Vizier (using the values presented for the flux as far as I can understand). I have two small related questions:

How does the CDS calculate the value from the uncertainty exactly? at least so that the one given by the CDS and the calculated one agree to the fourth decimal.

Is there a way in astropy or astroquery to retrieve the photometric uncertainty value from the catalog? Or do I have to calculate it as in the first question?

Thank you in advance.


texts: How would I calculate the length of the day of this planet?

texts:
Take the following planet:

Same distance from the main star as Earth
Main star is the same size as the sun
Planet is the size of Venus

How would I calculate the length of the day on this planet? If you need more information, please ask.


texts: Movement of the stars relative to my position

texts:
I live in the Pacific North West (Washington State, US). I have watched the skies for years, looking for constellations that I recognize and keeping track of them. One of them is Orion. As I have watched Orion I noticed that it travels East to West as I look to the south. It traverses the line that is the Milkey Way...

My question is. If the stars that make up the constellation of Orion move East to West from my position, looking up and South. Why does the Milky Way seem to stay in the same position? How is it that the stars that make up Orion move but the stars that make up the milky way appear to be fixed?

Thank you in advance for any input.


texts: Difference between quiet, enhanced and active networks

texts: For my research about the sun activity I need to process pictures of the sun. I want the pictures to show 3 stages the quiet, enhanced and active network. I couldn't find any information in the internet about how these 3 networks actually look and how they differ from each other. Can somebody explain this to me? What is showing on which picture?


texts: What is this web on the surface of the Sun?

texts:
I was going through my Social Media Feed and found the attached post too frequent. The caption reads this is the best image of our Sun. Just as an example, the Universe Today's This is the Highest Resolution Image Ever Taken of the Surface of the Sun

Why? What exactly are the black lines that appear to be like kind of a web, and will such patterns be observed if in case the star was not the Sun but some other star? Are they thought to be common?


texts: Do the orbits of planets change sometimes?

texts: Do planets sometimes wobble and get off their paths? What if an asteroid were to hit it?


texts: Where there is fire, there is always smoke. So why there isn't any smoke near the Sun?


texts: Has New Horizon's data updated Charon's orbital elements?

texts:
For years I've been fascinated with the mutually tide-locked bodies Pluto and Charon. In July 2012, The Astronomical Journal published an article The Orbit of Charon Is Circular by Buie, Tholen and Grundy. The authors thought that the eccentricity of Charon's orbit is very close to zero.

It seems to me their opinion is somewhat speculative given the low quality of images from Hubble. Has New Horizons data verified Charon's circular orbit? Is there an online source giving more precise orbital elements from New Horizons data?

I'm also interested in the obliquity of Pluto and Charon.


texts: Are Barlow lenses stackable for bright objects?

texts: I'm back to the refractor that I own, and I want to maximize the zoom capacity. I realized today that I own more than one barlow lens, and I was wondering if I could stack them together for looking at bright objects, like Jupiter. I'm aware that looking at anything else would probably be disastrous due to the stacked barlow lenses taking up more light. Is this an effective, or desirable way to approach the situation? What will happen if I do stack more than one barlow lens? Will it have the effect of an enhanced image, or will it do nothing but remove light?


texts: Is there a database of all known star names/identifiers?

texts:
I am looking for a list of all known identifiers used for stars. I'm not sure on the terminology. While I do want the "proper names", I also want whatever identifier is used for each star detected, even those without proper names. I am assuming there are far more of the latter.

If possible I would also like to have the coordinates and brightness (and any other interesting info) for each of these star. But just the list of identifiers would be great.

Does such data exist somewhere? Ideally I can just download a simple file (like a csv) and I can write my own software to process it without fiddling with some other software. However, it may be too large for that.

I'm not sure what to expect... 1 million entries?


texts: Why is magnitude of Beta Lyrae given as 30.0 in Henry Draper catalog?

texts:
Beta Lyrae (HD 174638) is ascribed visual magnitude 30.0 in the Henry Draper catalog. There are hundreds of other stars in the catalog also given magnitude 30.0. What was meant by these nominal magnitudes? Is there a resource in Vizier or elsewhere that provides corrected magnitudes?
https://vizier.u-strasbg.fr/viz-bin/VizieR-3?-source=III/135A/catalog&-out.max=50&-out.form=HTML%20Table&-out.add=_r&-out.add=_RAJ,_DEJ&-sort=_r&-oc.form=sexa


texts: How do we know which elements are in a galaxy spectra?

texts:
I know that each element has it's own distinct spectra (this is a good site that lists them: Periodic Table) but when looking at a spectra of a galaxy (or even a star) how do we match those elements to their distinct spectra? Looking at this image from Spectra in the Lab :

Some of the lines in the spectra overlap. The galaxies are also moving away, so it makes it more difficult to figure out the elements producing the emission/absprtion lines, right?

Also, each element usually has more than one absorption/emission line, so how can we say that just one line is the H-alpha line, or the K and H calcium lines, for example, as in this spectra:

(image from: http://spiff.rit.edu/classes/phys301/lectures/doppler/doppler.html)


texts: Understanding the statistical mechanics of Recombination Epoch ("Cosmology" Weinberg)

texts:
In Steven Weinberg's 'Cosmology' Chapter 2.3 (pg 113), he begins with a number density for some particle via the Maxwell-Boltzmann Formula:

While I have some familiarity with statistical mechanics, I can't figure out how this equation was derived. The spin states and fugacity are obvious enough for any quantum microcanonical ensemble. The $(2\pi\hbar)^{-3}$ and the integral however I am not sure where they come from. I am used to working with canonical ensembles in terms of partition functions and that doesn't seem to be the case here unless I am missing something.

Edit: I have also posted this question in physics.stackexchange because it is an application of physics (statistical mechanics).


texts: PHOEBE 2.2 Documentation, components and semi-major axis definitions

texts:
I'm looking at PHOEBE 2.2 Documentation, specifically at a tutorial about constraints

What are Constraints?
Constraints live in their own context of the Bundle, and many are created by default - either when you add a component or when you set the system hierarchy.
Let's look at all the existing constraints for our binary system...

and below that:

To see what all of these constraints do, see the 'Built-in Constraints' section below.
For now let's look at a single constraint by accessing a ConstraintParameter.
b['constraint']['primary']['mass']
<ConstraintParameter: {mass@primary@component} = (39.478418 * ({sma@binary@component} ** 3.000000)) / ((({period@binary@component} ** 2.000000) * (1.000000 + {q@binary@component})) * 2942.206217504418873431859537959099) (solar units) => 0.9988131358058302 solMass>
Here we see the equation used to derive the mass of the primary star from its orbit, as well as the current value
If we access the Parameter that it is constraining we can see that it is automatically kept up-to-date.

Could you explain to me the difference between the following please?

the semi-major axis of a component about the center of mass of its parent orbit
the semi-major axis of the parent orbit

The best would be a figure.


texts: Stellar core mass as a function of metallicity

texts:
Using Hurley's 2000 paper on Single Stellar Evolution, I have graphed the core mass of a star at three stages as a function of metallicity, Z, for a chosen ZAMS mass: at the beginning of the HG (MS), at the end of the Hertzprung-Gap (HG), and at the Base of the Asymptotic Giant Branch (BAGB).

All three functions have a cusp at Z = 0.0005 which I 1) do not understand physically and 2) wonder if it is incorrect? Is this expected? I would have expected that the core mass is monotonic in metallicity, Z. The cusp is present for any ZAMS mass chosen.

EDIT: The equations I use are...

Core mass at beginning of HG:
$Mc_{MS}$ is the product of equation 29 and the core mass at the end of the HG described below ($\rho$ is always $< 1$).

Core mass at end of HG:
$Mc_{HG}$ is, using equation 28 over the relevant mass range the HG ends at Helium Ignition, given by equation 44, where I solved for the time, $t$, in equation 35 and substituted that into equation 34 to find that $M_{c} = \Big(\frac{L}{D}\Big)^{1/5}$, and following the procedure given in the paragraph below equation 65 (where it references equation 44). I used equation 49b for the Luminosity here.

Core mass at BAGB:
$Mc_{BAGB}$ is equation 66.

I'm only interested in stars above $10 \odot{M}$, so I do not consider the possibility of a Blue Loop phase for simplicity.


texts: What explains the existence of energy/matter if it cannot be created or destroyed?

texts: If the energy/matter that was contained in the singularity before the Big Bang had always existed, and if it’s true that energy/matter cannot be created or destroyed, then what explains its existence?


texts:
A common notion is that supernovae create shock waves in interstellar gas, and can compress them to star forming densities.
How is that shockwave transmitted? Is it by ejected (sublight) particles, or is the radiation strong enough to move several solar masses of gas?


texts: If we were to see the Sun with our naked eyes from the Orion belt, would all planets be encompassed inside the star? Is this calculable?

texts:
When we see a star it looks much bigger in diameter to us than it really is, this picture (extracted from here) explains what I mean:

Notice that the point we see in the skynight, represented by the yellow outer circle, would actually encompass even planets orbiting the actual star, represented by the black dot in the center. In this case we see one planet inside the white circle.
My question is, can the effect of distance and brightness be measured as to know how far would we must be to watch a star that encompasses neptune?, the earth? or does it depend on different factors?


texts: Just how fast is a Fast Radio Burst thought to be?

texts:
According to Wikipedia;s Fast Radio Bursts; Frequencies and dispersion are recorded they

  The component frequencies of each burst are delayed by different amounts of time depending on the wavelength. This delay is described by a value referred to as a dispersion measure (DM). This results in a received signal that sweeps rapidly down in frequency, as longer wavelengths are delayed more.

The time between the arrival of the pulse at two different frequencies can be used to generate a kind of measure of distance, based on a dispersion constant. The measure does not have units of length, but of integrated electron density over the path from source to observer.

Using some fancy Fourier tricks one could first undo the $1/\nu^2$ delay and then try to reconstruct what the original pulse might have looked like before dispersion.

Has this been done? If so, how fast (narrow in time) might the original disturbance be? A millisecond? Less?


texts:
What would be the size of Sun as seen from surface of Pluto? It obviously depends on the distance and one would expect it to be very small. However, due to different optical phenomenon, the size of Sun seen on surface of Pluto could be larger. Can it can be estimated from the annular eclipse Sun-Pluto eclipse picture or other pictures taken by New Horizons probe ? Thanks for your insight.

The picture is also posted on this forum question.

Edit: From the answer given by @Conrad Turner below, Sun should be just pin point at that distance. However, when seen by New Horizons probe from behind Pluto, its size is seen to be almost as large as Pluto. The atmosphere of Pluto must be bending the light so much to make this happen.

Update: This picture was taken from a distance of 2 million km from Pluto. At this distance Sun will have an angular size of 0'49" (distance 5804550000 km, diameter 1392000 km), while Pluto has the angular size of 4'5" (distance 2000000 km, diameter 2372 km). However, the picture is showing Sun to have an angular size slightly larger than Pluto (or nearly the same size). Hence, high refraction or scattering or some other phenomenon are at work here.

Considering sizes and distances, if atmosphere of Pluto does nothing to light, the ring of Sun should have been visible around Pluto once probe was 10 million km beyond Pluto. However, the ring of Sun is seen in this picture when probe is  only 2 million km beyond Pluto. The scientists must have known the optical characteristics of Pluto's atmosphere and hence they got the probe early in proper position to take this picture.


texts:
Assume two similarly sized bodies tide-locked to one another orbiting a barycenter between the two. That barycenter orbits a star. Since the two are tide-locked, their sidereal rotational period and sidereal orbital period (about one another) are the same (right?). There's no axial tilts or eccentricities to screw with things either for the sake of simplicity.

What would I base the length of a conventional day on for a society living on these worlds?

My gut's telling me to calculate the sidereal period based of the barycentral semi-major axes (or would I use the separation between the two worlds still?) for each and then compute the synodic periods and just use those, but my brain's too stupid to tell whether that's going to actually be anything close to a "solar day". I don't need perfect - just close enough.


texts: Clarity on dust extinction terms and implementation: R_V, A_V, and E(B-V)

texts:
I have created a model galaxy spectrum using STARBURST99 and want to compare it to an observed spectrum of a particular galaxy. I understand that I need to redden my model spectrum to do this, so I am following the Cardelli et al. (1989) parameterization, which returns A(lambda)/A(V) for different wavelengths. I understand that the extinction correction in magnitudes is A_V, and I know that R_V = A_V/E(B-V). However I don't know how these terms relate to each other to apply them to my model spectrum (flux density).

I've read that the commonly adopted R_V value for the Milky Way is 3.1, but what is the E(B-V) value? If I want to apply extinction to a model galaxy for both the Milky Way AND the other galaxy's internal extinction, how would I go about this? For the external galaxy, I have a measured E(B-V) value to use (or does this only apply to the MW extinction?), but in this case I don't know what to use for R_V.


texts: Why is it strange that outer stars in a galaxy are travelling with the same speed as inner while the total mass is also increasing?

texts:
Astronomers say that it's strange that the outer stars of a galaxy are travelling with the same speed as the inner stars. Therefore they introduced dark matter to solve this strangeness. The theory that the inner stars 'have to' go faster is that because they are closer to the center so the gravity is more, like the earth is closer to the sun and faster than Neptune.

But what I wonder is, whether the fact that just the outer stars are attracted to more mass, because there is more mass of stars that are within the orbits of the outer stars, explains the equality? So in this view it is not so strange that they have the same speed because they are influenced by more gravity. Or is it that this extra influence is considered to be too little to explain the equality?


texts: How long until we cannot see any stars from other galaxies?

texts: Since all the galaxies are moving further away from each other at a faster and faster rate, eventually we won't be able to see stars from other galaxies. How long until we can't see stars from other galaxies?